Phase change memory adaptive programming

ABSTRACT

Some embodiments include methods and apparatus having a module configured to program a memory cell using a signal to cause the memory cell to have a programmed resistance value, to adjust a programming parameter value of the signal if the programmed resistance value is outside a target resistance value range, and to repeat at least one of the programming and the adjusting if the programmed resistance value is outside the target resistance value range, the signal including a different programming parameter value each time the programming is repeated.

This application is a divisional of U.S. patent application Ser. No. 13/932,942, filed Jul. 1, 2013, which is a divisional of U.S. patent application Ser. No. 12/905,534, filed Oct. 15, 2010, now issued as U.S. Pat. No. 8,477,523, which is a divisional of U.S. patent application Ser. No. 12/049,072, filed Mar. 14, 2008, now issued as U.S. Pat. No. 7,821,810, all of which are incorporated herein by reference in their entirety.

BACKGROUND

Computers and other electronic products, e.g., digital televisions, digital cameras, and cellular phones, often have a memory device to store data and other information. Some conventional memory devices may store information based on the amount of charges on a storage node of the memory cell. The storage node is usually made of semiconductor material such as silicon. Different values of the charge on the storage node may represent different values (e.g., binary values “0” and “1”) of a bit of information stored in the memory cell.

Other conventional memory devices (e.g., phase change memory devices) may store information based on a resistance state of a memory element of the memory cell. The memory element may have a material that can change between different phases (e.g., crystalline and amorphous phases) when programmed. Different phases of the material may cause the memory cell to have different resistance states with different resistance values. The different resistance states of a memory element may represent different values of the information stored in the memory.

In some phase change memory devices, configuring the memory cells to increase data storage density or programming the memory cells to meet some device performance specifications may pose a challenge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a memory device having a memory array with phase change memory cells according to an embodiment of the invention.

FIG. 2 show a partial block diagram of a memory device having a memory array including phase change memory cells with access components and memory elements according to an embodiment of the invention.

FIG. 3 through FIG. 5 show schematic diagrams of examples of different memory cells having different access components coupled to memory elements according to various embodiments of the invention.

FIG. 6 through FIG. 9 show a memory cell having a memory element with a structure configured to have various resistance states corresponding to various resistance values according to various embodiments of the invention.

FIG. 10 is a chart showing example resistance values for the memory element of FIG. 6 through FIG. 9 and example corresponding stored information values based on the example resistance values.

FIG. 11 through FIG. 14 show another memory cell having a memory element with another structure configured to have various resistance states corresponding to various resistance values according to various embodiments of the invention.

FIG. 15 is a graph of temperature versus time during a programming operation to reset a memory cell, such as one of the memory cells of FIG. 1 through FIG. 14, according to an embodiment of the invention.

FIG. 16 is a graph of current versus time of a signal used during the programming operation described with reference to FIG. 15.

FIG. 17 is a graph of current versus time of another signal used during the programming operation described with reference to FIG. 15.

FIG. 18 is a graph of temperature versus time during a programming operation to set a memory element of a memory cell, such as the memory cell of FIG. 1 through FIG. 14, according to an embodiment of the invention.

FIG. 19 is a graph of current versus time of signals that the programming operation described with reference to FIG. 18 may use.

FIG. 20 is a graph of a curve showing a relationship between resistance values and current values for use as target values during a programming operation according to an embodiment of the invention.

FIG. 21 is a graph of current versus time of different signals with pulses having adjustable amplitude values used during a programming operation according to an embodiment of the invention.

FIG. 22 is a graph showing resistance versus current during an example programming operation according to an embodiment of the invention.

FIG. 23 is a graph showing various adjusted current values based on an initial target current value used during the example programming operation associated with FIG. 22.

FIG. 24 is a graph showing resistance versus current during an example programming operation using adjusted current values based on slope calculations according to an embodiment of the invention.

FIG. 25 is a graph showing various adjusted current values used during the example programming operation associated with FIG. 24.

FIG. 26 is a graph of a curve showing a relationship between resistance values and transition time values for use as target values during a programming operation according to an embodiment of the invention.

FIG. 27 is a graph showing different signals with pulses having adjustable transition time values used during a programming operation according to an embodiment of the invention.

FIG. 28 is a graph showing resistance versus transition time during an example programming operation according to an embodiment of the invention.

FIG. 29 is a graph showing various adjusted transition time values used based on an initial target transition time value used during the example programming operation associated with FIG. 28.

FIG. 30 is a flow diagram showing methods of operating a device according to an embodiment of the invention.

FIG. 31 is a table showing a relationship between target resistance values and corresponding target current values for the methods associated with FIG. 30.

FIG. 32 is a table showing a relationship between target resistance values and corresponding target transition time values for the methods associated with FIG. 30.

FIG. 33 is a flow diagram showing methods of operating a device including amplitude value adjustment according to an embodiment of the invention.

FIG. 34 is a flow diagram showing methods of operating a device including transition time value adjustment according to an embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 shows a block diagram of a memory device 101 having a memory array 102 with phase change memory cells 100 according to an embodiment of the invention. Memory cells 100 may be arranged in rows and columns along with lines 104 (e.g., wordlines having signals WL0 through WLm) and lines 106 (e.g., bit lines having signals BL0 through BLn). Memory device 101 may use lines 104 and lines 106 to transfer information with memory cells 100. Row decoder 107 and column decoder 108 may decode address signals A0 through AX on lines 109 (e.g., address lines) to determine which memory cells 100 are to be accessed. A sense amplifier circuit 110 may operate to determine the value of information read from memory cells 100 and provide the information in the form of signals to lines 106. Sense amplifier circuit 110 may also use the signals on lines 106 to determine the value of information to be written to memory cells 100. Memory device 101 may include circuitry 112 to transfer information between memory array 102 and lines (e.g., data lines) 105. Signals DQ0 through DQN on lines 105 may represent information read from or written into memory cells 100. Lines 105 may include nodes within memory device 101 or pins (or solder balls) on a package where memory device 101 may reside. Other devices external to memory device 101 (e.g., a memory controller or a processor) may communicate with memory device 101 through lines 105, 109, and 120.

Memory device 101 may perform memory operations such as a read operation to read information from memory cells 100 and a programming operation (sometime referred to as write operation) to program (e.g., write) information into memory cells 100. A memory control unit 118 may control the memory operations based on control signals on lines 120. Examples of the control signals on lines 120 may include one or more clock signals and other signals to indicate which operation, (e.g., a programming or read operation) memory device 101 may perform. Other devices external to memory device 101 (e.g., a processor or a memory controller) may control the values of the control signals on lines 120. Specific values of a combination of the signals on lines 120 may produce a command (e.g., programming or read command) that may cause memory device 101 to perform a corresponding memory operation (e.g., programming or read operation).

Each of memory cells 100 may be programmed to store information representing a value of a single bit or a value of multiple bits such as two, three, four, or another number of bits. For example, each of memory cells 100 may be programmed to store information representing a binary value “0” or “1” of a single bit. In another example, each of memory cells 100 may be programmed to store information representing a value of multiple bits, such as one of four possible values “00”, “01”, “10”, and “11” of two bits, one of eight possible values “000”, “001”, “010”, “011”, “100”, “101”, “110” and “111”, or one of other values of another number of multiple bits.

Memory device 101 may receive a supply voltage, including supply voltage signals Vcc and Vss, on lines 130 and 132, respectively. Supply voltage signal Vss may operate at a ground potential (e.g., having a value of approximately zero volts). Supply voltage signal Vcc may include an external voltage supplied to memory device 101 from an external power source such as a battery or an alternating-current to direct-current (AC-DC) converter circuitry.

Circuitry 112 of memory device 101 may include a select circuit 115 and an input/output (I/O) circuit 116. Select circuit 115 may respond to signals SEL1 through SELn to select the signals on lines 106 and 113 that may represent the information read from or programmed into memory cells 100. Column decoder 108 may selectively activate the SEL1 through SELn signals based on the A0 through AX address signals on lines 109. Select circuit 115 may select the signals on lines 106 and 113 to provide communication between memory array 102 and I/O circuit 116 during read and programming operations.

Memory device 101 may include a non-volatile memory device and memory cells 100 may include non-volatile memory cells such that memory cells 100 may retain information stored thereon when power (e.g., Vcc or Vss, or both) is disconnected from memory device 101. For example, memory device 101 may include a phase change memory device such that each of memory cells 100 may include a memory element having a material in which at least a portion (e.g., programmable portion) of the material may be programmed to cause the portion to change between different phases, such as between a crystalline phase (which is sometimes referred to as a crystalline state) and an amorphous phase (which is sometimes referred to as an amorphous state). Each of memory cells 100 may have a resistance state corresponding to a resistance value when the memory cell is programmed. Different resistance values may represent different values of information programmed in each of memory cells 100.

Memory device 101 may perform a programming operation when it receives (e.g. from an external processor or a memory controller) a programming command and value of information to be programmed into one or more of selected memory cells among memory cells 100. Based on the value of the information, memory device 101 may program the selected memory cells to cause them to have appropriate resistance values to represent the values of the information.

Memory device 101 may include a storage area 144 to store programming parameter values and selectively use these values during a programming operation. Memory device 101 may store the programming parameter values in the form of a table 155 in storage area 144. The programming parameter values may include amplitude value, transition time value, or both, of a pulse of the signal that memory device 101 may use in a programming operation. The amplitude value may correspond to current amplitude value (e.g., in ampere unit) of the pulse or voltage amplitude value (e.g., in volt unit) of the pulse to be used in a programming operation. The transition time value may correspond to a rise time (in time unit such as nanosecond) of a rising edge or a fall time (in time unit such as nanosecond) of a falling edge of the pulse to be used in a programming operation. The programming parameter values may also include offset values that memory device 101 may use to determine an acceptable range for a target resistance value to be programmed into a memory cell.

One skilled in the art may recognize that memory device 101 may include other components that are not shown to help focus on the embodiments described herein.

Memory device 101 may include devices, memory cells, and programming operations similar to or identical to those described below with reference to FIG. 2 through FIG. 34.

FIG. 2 show a partial block diagram of a memory device 201 having a memory array 202 including phase change memory cells 200 with access components 211 and memory elements 222 according to an embodiment of the invention. Memory array 202 may correspond to memory array 102 of FIG. 1. As shown in FIG. 2, memory cells 200 may be arranged in rows 230, 231, and 232 along with lines having signals WL0, WL1, and WL2, and columns 240, 241, and 242 along with lines having signals BL0, BL1, and BL2. Access components 211 may turn on (e.g., by using appropriate values of signals WL0, WL1, and WL2) to allow access to memory elements 222 to read information (e.g., measure a resistance value) from memory elements 222 or program information into memory elements 222 (e.g., causing memory elements 222 to have specific resistance value).

FIG. 3 through FIG. 5 show schematic diagrams of examples of different memory cells 300, 400, and 500 having different access components 311, 411, and 511 coupled to memory elements 333, 444, and 555 according to various embodiments of the invention. A line with a signal WL and a line with a signal BL in FIG. 3 through FIG. 5 may correspond to one of lines 104 and one of lines 106 of FIG. 1, respectively. FIG. 3 through FIG. 5 show examples of access components 311, 411, and 511 including a metal-oxide-semiconductor field-effect transistor (MOSFET), a bipolar junction transistor (BJT), and a diode, respectively. Memory cells 300, 400, and 500 may include other types of access components.

As shown in FIG. 3 through FIG. 5, each of memory elements 333, 444, and 555 may couple between two electrodes, such as electrodes 351 and 352 (FIG. 3), electrodes 451 and 452 (FIG. 4), or electrodes 551 and 552 (FIG. 5). FIG. 3 through FIG. 5 schematically show electrodes 351, 352, 451, 452, 551, and 552 as dots. Structurally, each of these electrodes may include a conductive material. In FIG. 3 through FIG. 5, access components 311, 411, and 511 may enable signals (e.g., voltage or current) to be transferred to and from memory elements 333, 444, and 555 via electrodes 351, 352, 451, 452, 551, and 552 during a programming on read operation.

For example, a programming operation may use signal WL to turn on access components 311, 411, and 511, and then apply a current (e.g., programming current) through memory elements 333, 444, and 555. The current causes at least a portion of the material of memory elements 333, 444, and 555 to heat up and melt. After the material melts, the programming operation may allow the material to rapidly cool down. These heat and cool actions may change the phase of the material, such as from a crystalline phase before the programming operation to an amorphous phase after the programming operation. The phase change may be reversible (e.g., changing from an amorphous phase to a crystalline phase). Different phases of the material may cause memory elements 333, 444, and 555 to have different resistance states with different resistance values, which correspond to different values of the information that is being stored in memory elements 333, 444, and 555.

In another example, a read operation may use signal WL to turn on access components 311, 411, and 511, and then apply a current (e.g., read current) through memory elements 333, 444, and 555. The read operation may measure the resistance of memory cells 300, 400, and 500 based on a read voltage to determine the corresponding value of information stored therein. For example, in each of memory cells 300, 400, and 500, a different resistance value may provide a different value (e.g., current or voltage value) on signal BL when the current passes through memory elements 333, 444, and 555. Other circuitry of the memory device (e.g., circuit such as I/O circuit 116 of FIG. 1) may use signal BL to measure the resistance value of memory elements 333, 444, and 555 to determine the value of the information.

The current used during a read operation may have a value different from the current used during a programming operation. For example, in a programming operation, the value of the signal (e.g., signals from line BL in FIG. 3 or FIG. 4 or from line WL in FIG. 5) that creates a current flowing through the memory element may be sufficient to cause the material of at least a portion of the memory element to change between different phases to alter the resistance value of the memory element based on the value of the information to be stored in memory elements 333, 444, and 555. In a read operation, the value of the signal (e.g., signals from line BL in FIG. 3 or FIG. 4 or from line WL in FIG. 5) that creates a current flowing through the memory element may be sufficient to create the current but insufficient to cause any portion of the memory element to change between different phases so that the value of the information stored in the memory element may remain unchanged in the read operation.

Memory cells 100, 200, 300, 400, and 500 of FIG. 1 through FIG. 5 may include a memory cell similar to or identical to one or more of the memory cells described below with reference to FIG. 6 through FIG. 34.

FIG. 6 through FIG. 9 show a memory cell 600 having a memory element 666 with a structure configured to have various resistance states 633, 733, 833, and 933 corresponding to various resistance values R0, R1, R2, and R3 according to various embodiments of the invention. As shown in FIG. 6 through FIG. 9, memory cell 600 may include electrodes 651 and 652 coupled to memory element 666. Memory cell 600 may also include other components, such as an access component that may be similar to or identical to access component 211, 311, 411, or 511 (FIG. 2 through FIG. 5). FIG. 6 through FIG. 9 omit the other components of memory cell 600 to help focus on the embodiments discussed herein. Further, for clarity, FIG. 6 through FIG. 9 show memory element 666 with cross-section lines (shading lines) and electrodes 651 and 652 without cross-section lines. Electrodes 651 and 652 may correspond to electrodes 351 and 352 (FIG. 3), electrodes 451 and 452 (FIG. 4), or electrodes 551 and 552 (FIG. 5), which are schematically shown as dots in FIG. 3 through FIG. 5. In FIG. 6 through FIG. 9, memory element 666 may include the same material for portions 601, 602, and 603. The material may be configured to change between multiple phases, e.g., between crystalline and amorphous phases. Memory element 666 may include a phase change material. Some phase change materials may include chalcogenide materials with various combinations of germanium (Ge), antimony (Sb), tellurium (Te), and other similar materials. Examples of phase change materials may include binary combinations such as germanium telluride (GeTe), indium selenide (InSe), antimony telluride (SbTe), gallium antimonide (GaSb), indium antimonide (InSb), arsenic telluride (AsTe), aluminum telluride (AlTe); ternary combinations such as germanium antimony telluride (GeSbTe), tellurium germanium arsenide (TeGeAs), indium antimony telluride (InSbTe), tellurium tin selenide (TeSnSe), germanium selenium gallide (GeSeGa), bismuth selenium antimonide (BiSeSb), gallium selenium telluride (GaSeTe), tin antimony telluride (SnSbTe), indium antimony germanide (InSbGe); and quaternary combinations such as tellurium germanium antimony sulfide (TeGeSbS), tellurium germanium tin oxide (TeGeSnO), and alloys of tellurium germanium tin gold, palladium tellurium germanium tin, indium selenium titanium cobalt, germanium antimony tellurium palladium, germanium antimony tellurium cobalt, antimony tellurium bismuth selenium, silver indium antimony tellurium, germanium antimony selenium tellurium, germanium tin antimony tellurium, germanium tellurium tin nickel, germanium tellurium tin palladium, germanium tellurium tin platinum. Among the phase change materials listed herein, some may provide an appropriate choice over the others, depending in part on the application of the device. For example, a compound of germanium (Ge), antimony (Sb), and telluride (Te) may be an appropriate choice for a phase change memory device, in part, because of its relatively quick switching speed (e.g., a few nanoseconds) between different resistance states. The compound of germanium (Ge), antimony (Sb), and telluride (Te) may have a formula Ge₂Sb₅Te₅. Most of the material compositions in this description list only the component elements. The relative amount of each component element in each of these material compositions is not limited to a particular value.

As shown in FIG. 6 through FIG. 9, memory element 666 may include a portion 601 directly contacting electrode 651, a portion 602 directly contacting electrode 652, and a portion 603 between portions 601 and 602. FIG. 6 shows portion 603 being located at a general area indicated by a broken circle to indicate that portion 603 may include a part of portion 601, a part of portion 602, or both. Portion 603 of memory element 666 may be referred to as a programmable portion (or programmable volume). A programming operation may program memory cell 600 to one of multiple possible resistance states 633, 733, 833, and 933 corresponding to one of resistance values (in ohm unit) R0, R1, R2, and R3. The value of the information stored in memory element 666 may be based on which one of resistance values R0, R1, R2, and R3 that memory element 666 may have.

FIG. 6 shows an example where memory element 666 may have resistance value R0 in which the material at portions 601, 602, and 603 has the same crystalline phase 613. A programming operation may program memory cell 600 to cause at least a part of portion 603 to “amorphize” (e.g., changes from a crystalline phase to an amorphous phase), resulting in an amorphized region 713, 813, or 913. As shown in FIG. 6 through FIG. 9, portion 603 (either before or after programming) is isolated from electrode 651 by portion 601 of memory element 666, and is isolated from electrode 652 by portion 602 of memory element 666. Isolating portion 603 from electrodes 651 and 652 may reduce heat from being directly transferred from portion 603 to electrodes 651 and 652 during programming of memory element 666, thereby decreasing potential heat loss. Thus, programming time may be reduced, a relatively lower amount of programming current may be used, and power may be saved. FIG. 7 through FIG. 9 show amorphized regions 713, 813, and 913 having different region sizes. For example, amorphized region 713 may have size that is smaller than of amorphized region 813. Amorphized region 813 may have a size that is smaller than that of amorphized region 913. Different sizes of the amorphized region of portion 603 may cause memory cell 600 to have a different resistance value (e.g., R1, R2, or R3) to represent different values of information.

FIG. 10 is a chart 1000 showing example resistance values R0, R1, R2, and R3 of the memory element 666 of FIG. 6 through FIG. 9 and example corresponding stored information values based on the example resistance values. Memory cell 600 may be configured to store information representing a value of multiple bits (e.g., two, three, four, or other bits). Chart 1000 of FIG. 10 shows an example where each of four resistance values R0, R1, R2, and R3 may be assigned to a unique value of four possible values “00”, “01”, “10”, and “11” of two bits.

FIG. 11 through FIG. 14 show another memory cell 1100 having a memory element 1111 with another structure configured to have various resistance states 1133, 1233, 1333, and 1433 corresponding to various resistance values R0, R1, R2, and R3 according to various embodiments of the invention. Memory cell 1100 may also include other components, such as an access component that may be similar to or identical to access component 211, 311, 411, or 511 (FIG. 2 through FIG. 5). In FIG. 11 through FIG. 14, memory element 1111 may include material similar or identical to the material of memory element 666 of FIG. 6 through FIG. 9.

In FIG. 11 through FIG. 14, memory element 1111 may include a portion 1101 directly contacting electrode 1151, a portion 1102 directly contacting electrode 1152, and a portion 1103 between portions 1101 and 1102. FIG. 11 shows portion 1103 being located at a general area indicated by a broken circle to indicate that portion 1103 may include a part of portion 1101, a part of portion 1102, or both. Portion 1103 of memory element 1111 may be referred to as a programmable portion. A programming operation may program memory cell 1100 to one of multiple possible resistance states 1133, 1233, 1333, and 1433 corresponding to one of resistance values R0, R1, R2, and R3. The value of the information stored in memory cell 1100 may be based on which one of resistance values R0, R1, R2, and R3 that memory element 1111 may have. As shown in FIG. 11, memory cell 1100 may have resistance value R0 in which the material at portions 1101, 1102, and 1103 has the same crystalline phase 1113, and resistance value R1, R2, and R3 in which portion 1103 may have amorphized regions 1213, 1313, and 1413, respectively, with different region sizes that may correspond to information values, e.g., “00,” “01,” “10,” and “11” of FIG. 10, that may be stored in memory cell 1100.

The description herein includes ways to program a memory cell of a phase change memory device such as memory device 101 or 201 (FIG. 1 or FIG. 2) in which the memory device may include a memory cell with a memory element having a structure such as the structure of memory element 666 (FIG. 6 through FIG. 9) and memory element 1111 (FIG. 11 through FIG. 14), or other structure.

The programming operation may include reset activities (occasionally called “reset” or “resetting”) and set activities (occasionally called “set” or “setting”). The reset activities may change the memory cell to a resistance state in which the material of a programmable portion of the memory element may have one phase (e.g., amorphous phase) and material of the other portions of the memory element have another phase (e.g., crystalline phase). For example, the reset activity may change memory cell 600 (FIG. 6) from resistance state 633 corresponding to resistance value R0 to a resistance state 733, 833, or 933 (FIG. 7 through FIG. 9) corresponding to one of resistance values R1, R2, or R3. Thus, each of resistance states 733, 833, and 933 (FIG. 7) may be called a “reset” resistance state. Each of resistance values R1, R2, and R3 may be called a “reset” resistance value. The set activities may change the memory cell from a reset resistance state to a “set” resistance state at which the material of memory element 666 may have the same phase (e.g., crystalline phase), such as a resistance state 633 corresponding to resistance value R0 in FIG. 6. Thus, resistance state 633 may be called a “set” resistance state. Resistance value R0 may be called a “set” resistance value. The reset and set activities of the programming operations described herein may includes activities of the programming operations described below with reference to FIG. 15 through FIG. 34.

FIG. 15 is a graph of temperature versus time during a programming operation to reset a memory cell, such as one of memory cell 100, 200, 300, 400, 500, 600, and 1100 of FIG. 1 through FIG. 14, according to an embodiment of the invention. As shown in FIG. 6 through FIG. 9 and FIG. 11 through FIG. 14, resistance values R0, R1, R2, and R3 may depend on the characteristics of the material at the programmable portion (e.g., portion 603 or 1103) of the memory cell. For example, different size of the amorphized region (713, 813, or 913 of FIG. 7 through FIG. 9) may result in a different resistance value (e.g., R1, R2, or R3).

As shown in FIG. 15, to reset the memory cell so that the programmable portion of the memory cell may have an amorphized region, the programming operation may use a signal to cause the material of at the programmable portion to heat and allow the temperature of material at the programmable portion to rise and exceed the material crystalline temperature Tc and melting point temperature Tm. The material at the programmable portion may melt. The programming operation may allow the material to cool (or rapidly cool) after it melts. As a result, the material at the programmable portion may amorphize and result in an amorphized region. The size of the amorphized region may depend on signal values of a signal used during the programming operation. The signal may include a pulse. The signal values may include an amplitude value of the pulse, the transition time value of the pulse (e.g., transition time value of an edge of the pulse), or both.

FIG. 16 is a graph of current versus time of a signal used during the programming operation described with reference to FIG. 15. As shown in FIG. 16, signal 1600 may include one or more pulses, such as a pulse 1601 having a square (or substantially square) wave shape where a transition time value (e.g., pulse rise time value) of edge 1611 and transition time value (e.g., pulse fall time value) of edge 1612 may be considerably less than the duration of pulse 1601. Pulse 1601 may also have an amplitude value 1680, which may correspond to a current value I_(RESETX). Different values of amplitude value 1680 (e.g., different current values) may cause a programmable portion of a memory cell to amorphize to different amorphized regions with different sizes corresponding to different resistance values (e.g., R1, R2, and R3).

FIG. 17 is a graph of current versus time of another signal 1700 used during the programming operation described with reference to FIG. 15. As shown in FIG. 17 signal 1700 may include one or more pulses, such as a pulse 1701 having an amplitude value 1780, which may correspond to a current value I_(RESETY). Pulse 1701 may also have edges 1711 and 1712. As shown in FIG. 17, unlike pulse 1601 of FIG. 16, pulse 1701 of FIG. 17 may have a shape different from a square wave shape. A transition time value (e.g., pulse rise time value) of edge 1711 and a transition time value (e.g., pulse fall time value) 1720 of edge 1712 are unequal. Different values of transition time value 1720 may cause a programmable portion of a memory cell to amorphize with different crystalline composition corresponding to different resistance values (e.g., R1, R2, and R3), with greater transition time value 1720 resulting in greater crystalline composition and lower resistance value.

As described above with reference to FIG. 16 and FIG. 17, to reset a memory cell to cause it to change from one resistance value (e.g., R0) to another resistance value (e.g., R1, R2, or R3), the programming operation may use a signal having a pulse with a square wave shape such as signal 1600 of FIG. 16, or a different signal such as signal 1700 of FIG. 17 with unequal rise time and fall time values.

FIG. 18 is a graph of temperature versus time during a programming operation to set a memory element of a memory cell, such as the memory cell 100, 200, 300, 400, 500, 600, and 1100 of FIG. 1 through FIG. 14, according to an embodiment of the invention. As shown in FIG. 18, the programming operation may use a signal to cause the material at the programmable portion to heat and allow the temperature of the material at the programmable portion to rise above its crystalline temperature Tc but below its melting point temperature Tm. The programming operation may hold the temperature of the material for an interval and then allow it to cool. As result, the programmable portion may “recrystallize” (e.g., change from an amorphous phase to a crystalline phase) such that the material of the programmable portion and other portions (e.g., portions 601, 602, and 603 of FIG. 6) may have the same phase, causing the memory cell to have a resistance value such as resistance value R0 of FIG. 6.

FIG. 19 is a graph of current versus time of signals 1900 and 1902 that the programming operation described with reference to FIG. 18 may use. Either signal 1900 or 1902 may be used in the programming operation. As shown in FIG. 19, signal 1900 may include a pulse 1901 having a square (or substantially square) wave shape with an amplitude value corresponding to a current value I_(SET), which is less than a current value I_(RESET). Signal 1902 may include a pulse 1903 having a non-square shape with a fall time value 1905. FIG. 19 shows pulse 1903 having an amplitude value corresponding to a current value I_(RESET) and fall time value 1905 as an example. The amplitude of pulse 1903 and the fall time value of pulse 1905 may vary. Current value I_(RESET) of FIG. 19 may correspond to current value I_(RESETX) (FIG. 16) or I_(RESETY) (FIG. 17).

FIG. 20 is a graph of a curve 2000 showing a relationship between resistance values R0, R1, R2, and R3 and current values I_(SET), I_(RESET1), I_(RESET2), and I_(RESET3) for use as target values during a programming operation according to an embodiment of the invention. The device associated with FIG. 20 may store (e.g., in form of values in a table) the relationship presented by curve 2000 in a storage area of the device, such as a storage area similar to or identical to storage area 144 of memory device 101 of FIG. 1. In determining the relationship, the device associated with FIG. 20 may include activities to characterize (e.g., program and then read) one or more selected memory cells during an interval such as during device initialization. For example, the device may program (e.g., during device initialization) one or more selected memory cells using different signals with different pulse amplitude values corresponding to different programming current values (e.g., I_(SET), Ia, I_(RESET1), Ib, I_(RESET2), Ic, I_(RESET3), and Id of FIG. 20). Then, the device may read the selected memory cells (e.g., during device initialization) to obtain their resistance values (e.g., R0, Ra, R1, Rb, R2, Rc, R3, and Rd of FIG. 20) corresponding to the different programming current values. Thus, many different programming current values may be used to construct the relationship (presented by curve 2000), which may or may not include the resistance values R0, R1, R2, and R3. If resistance values R0, R1, R2, and R3 are not included, they may be extrapolated from the relationship. A programming operation (e.g., after the initialization) may use this relationship (e.g., values from the table) to program a memory cell.

In FIG. 20, each of resistance values R0, R1, R2, and R3 may be called a target resistance value, occasionally referred to as R_(TAR). R_(TAR) is the intended resistance value that the memory cell is to be programmed. The programming operation may determine R_(TAR) based on the value of information to be stored in the memory cell. Each of current values I_(SET), I_(RESET1), I_(RESET2), and I_(RESET3) may be called a target current value (occasionally referred to as I_(TAR)). During a programming operation, e.g., after the initialization, the programming operation may select one of current values I_(SET), I_(RESET1), I_(RESET2), and I_(RESET3) to be I_(TAR), depending on which one the resistance values R0, R1, R2, and R3 is determined to be R_(TAR). Thus, I_(TAR) is the current value that the programming operation may initially select to start the programming of a memory cell. During programming, the programming operation may successively adjust I_(TAR) to program the memory cell until the memory cell has a resistance value equal to R_(TAR) or to a value within an acceptable range of R_(TAR).

The programming operation may select different values of I_(TAR) to program different selected memory cells if the selected memory cells are to be programmed with different information values. For example, based on the relationship between resistance values R0, R1, R2, and R3 and current values I_(SET), I_(RESET1), I_(RESET2), and I_(RESET3), the programming operation may select I_(TAR)=I_(RESET1) (and successively adjust I_(RESET1)) to program a first memory cell if the information to be stored in the first memory cell corresponds to resistance value R1. Then, programming operation may select I_(TAR)=I_(RESET2) (and successively adjust I_(RESET2)) to program a second memory cell if the information to be stored in the second memory cell corresponds to resistance value R2.

FIG. 21 is a graph of current versus time of different signals with pulses 2100, 2101, 2102, and 2103 having adjustable amplitude values 2181, 2182, and 2183 used during a programming operation according to an embodiment of the invention. Amplitude values 2181, 2182, and 2183 may correspond to current values I_(RESET1), I_(RESET2), and I_(RESET3), respectively, which are greater than a current value I_(SET). A programming operation may use pulses 2100, 2101, 2102, and 2103 to cause a memory cell to have corresponding resistance values R0, R1, R2, and R3, respectively. Current values I_(SET), I_(RESET1), I_(RESET2), and I_(RESET3) of FIG. 21 may correspond to those of FIG. 20.

As shown in FIG. 21, the programming operation may adjust amplitude values 2181, 2182, and 2183 to obtain adjusted amplitude values 2191, 2192, and 2193, respectively. Each of adjusted amplitude values 2191, 2192, and 2193 may include a value that is less than or greater than its corresponding amplitude value by an adjusted amplitude amount. For example, as shown in FIG. 21, adjusted amplitude value 2192 may be less than or greater than amplitude value 2182. Depending on the value of R_(TAR), the programming operation may select one of amplitude values 2181, 2182, and 2183 as an initial target amplitude value and may adjust the selected initial target amplitude value during programming to program the memory cell until its programmed resistance value (occasionally referred to as R_(PRG)) is equal to R_(TAR) (e.g., R1, R2, or R3) or is within an acceptable range of R_(TAR).

FIG. 22 is a graph showing resistance versus current during an example programming operation according to an embodiment of the invention. As described above with reference to FIGS. 20 and 21, R_(TAR) may represent a target resistance value to be programmed to a memory cell. R_(TAR) in FIG. 22 may correspond to one of R1, R2, and R3 of FIG. 20 and FIG. 21. For the purposes of describing the example programming operation associated with FIG. 22, R_(TAR) is assumed to be resistance value R2. The programming operation may finish programming the memory cell when the memory cell has a resistance value within a target resistance value range of R_(TAR) (occasionally referred to as the R_(TAR) range). As shown in FIG. 22, the R_(TAR) range may include an upper resistance value equal to R_(TAR) plus an offset value OFFSET_(H), and a lower resistance value equal to R_(TAR) minus an offset value OFFSET_(L). OFFSET_(H) and OFFSET_(L) may have an equal value. OFFSET_(H) may have a value (e.g., less than half of a difference between R_(TAR), e.g., R2, and a next higher target resistance value, e.g., R3) such that the upper resistance value (R_(TAR)+OFFSET_(H)) of FIG. 22 may not overlap the lower resistance value (e.g., R3 minus OFFSET_(L)) of another target resistance value range. Similarly, OFFSET_(L) may have a value (e.g., less than half of a difference between R_(TAR), e.g., R2, and a next lower target resistance value, e.g., R1) such that the lower resistance value (R_(TAR)−OFFSET_(L)) of FIG. 22 may not overlap the upper resistance value (e.g., R1 plus OFFSET_(H)) of another target resistance value range.

In FIG. 22, R_(PRG.A), R_(PRG.B), R_(PRG.C), R_(PRG.D), R_(PRG.E), and R_(PRG.F) may represent various programmed resistance values that the memory cell may have during various repeated programming. The programming operation may repeat programming (e.g., resetting) the memory cell more than one time in an attempt to cause the programmed resistance value to be within the R_(TAR) range.

As described above with reference to FIG. 20, based on a relationship between resistance value and current value, a programming operation may use a signal with an initial target amplitude value corresponding to current value I_(RESET1), I_(RESET2), or I_(RESET3) to reset the memory cell to resistance value R1, R2, or R3. Here, in the example of FIG. 22, since R_(TAR) is assumed to be resistance value R2, the programming operation may use a signal having a pulse with an initial target amplitude value corresponding to target current value I_(TAR)=I_(RESET2) to program the memory cell so that R_(TAR) (or R2 in this example) may be programmed into the memory cell.

The programming operation associated with FIG. 22 may include the following activities. The programming operation may initially use a current value (e.g., I_(RESET2)) to program the memory cell. If the programmed resistance value R_(PRG) is outside the R_(TAR) range, then the programming operation may successively adjust the current value (e.g., by increasing or decreasing the current value by an adjusted current amount such as Δi, ½ Δi, ¼ Δi, ⅛ Δi, or 1/16 Δi) to obtain different adjusted current values. Adjusted current amounts such as Δi, ½ Δi, ¼ Δi, ⅛ Δi, and 1/16 Δi) may be called adjusted parameter amounts. After each adjustment, the programming operation may use the adjusted current values and repeat programming the memory cell until R_(PRG) is within the R_(TAR) range.

For example, in FIG. 22, after being programmed using a current value I_(RESET2), the memory cell may have R_(PRG.A), which is outside the R_(TAR) range and less than R_(TAR). Thus, the programming operation may adjust I_(RESET2) and repeat the programming until the memory cell has the programmed resistance value (e.g., R_(PRG.F)) that is within the R_(TAR) range.

As shown in FIG. 22, the memory cell may have R_(PRG.A), which is outside the R_(TAR) range, after being programmed using an initial current value I_(RESET2). To repeat programming, the programming operation may adjust I_(RESET2) by increasing I_(RESET2) by Δi, and then repeat programming the memory cell using an adjusted current value I_(RESET2B)=I_(RESET2)+Δi.

After programming using I_(RESET2B), the memory cell may have R_(PRG.B), which is still outside the R_(TAR) range and less than R_(TAR). Thus, the programming operation may adjust I_(RESET2B) by increasing I_(RESET2B) by Δi, and then repeat programming the memory cell using an adjusted current value I_(RESET2C)=I_(RESET2B)+Δi. The programming operation may keep increasing the current value by Δi until the programmed resistance value is either within the R_(TAR) range (then the programming operation may finish programming the memory cell) or outside the R_(TAR) range but greater than R_(TAR) (then the programming operation may continue but with an adjusted current amount less than Δi).

In FIG. 22, after programming using I_(RESET2C), the memory cell may have R_(PRG.C), which is still outside the R_(TAR) range but greater than R_(TAR). Thus, the programming operation may adjust I_(RESET2C) by decreasing I_(RESET2C) by ½ Δi, and then repeat programming the memory cell using an adjusted current value I_(RESET2D)=I_(RESET2C)−½ Δi.

After programming using I_(RESET2D), the memory cell may have R_(PRG.D), which is still outside the R_(TAR) range and greater than R_(TAR). Thus, the programming operation may adjust I_(RESET2D) by decreasing I_(RESET2D) by ¼ Δi, and then repeat programming the memory cell using an adjusted current value I_(RESET2E)=I_(RESET2D)−¼ Δi.

After programming using I_(RESET2E), the memory cell may have R_(PRG.E), which is still outside the R_(TAR) range and greater than R_(TAR). Thus, the programming operation may adjust I_(RESET2E) by decreasing I_(RESET2E) by ⅛ Δi, and then repeat programming the memory cell using an adjusted current value I_(RESET2F)=I_(RESET2E)−⅛ Δi. After programming using I_(RESET2F), the memory cell may have R_(PRG.F), which is within the R_(TAR) range. Since R_(PRG.F) is within the R_(TAR) range, the programming operation may finish programming the memory cell.

As described above, the programming operation may increase the previous current value by an adjusted current amount (e.g., Δi) if after being programmed, the memory cell has a programmed resistance value (e.g., R_(PRG.A) or R_(PRG.B)) that is outside the R_(TAR) range and less than R_(TAR). In the opposite, the programming operation may decrease the previous current value by an adjusted current amount (e.g., ½ Δi, ¼ Δi, or ⅛ Δi) if after being programmed, the memory cell has a resistance value (e.g., R_(PRG.C), R_(PRG.D) or R_(PRG.F)) that is outside the R_(TAR) range and greater than R_(TAR). Since the adjusted current amount (Δi, ½ Δi, ¼ Δi, ⅛ Δi, or 1/16 Δi) may be different (e.g., varied) when the current value is adjusted, the adjusted current amount may include a variable current amount.

In FIG. 22, Δi may be called a selected current amount. The device associated with FIG. 22 may select the value of Δi to be any appropriate value. However, to improve the chance of a programmed resistance to be within the R_(TAR) range or to reduce the number of repeated programming, for example, the device may select the value of Δi based on a difference in values between two adjacent reset current values from a resistance value versus current value curve, such as curve 2200 of FIG. 22. For example, the device may select the value of Δi to be equal to the difference between I_(TAR)+OFFSET′_(H) and I_(TAR)−OFFSET′_(L) or less than (e.g., one-half of) the difference between I_(TAR)+OFFSET′_(H) and I_(TAR)+OFFSET′_(L). I_(TAR)+OFFSET′_(H) and I_(TAR)−OFFSET′_(L) may include current values of the pulses that are used to obtain R_(TAR)+OFFSET_(H) and R_(TAR)−OFFSET_(L), respectively.

FIG. 23 is a graph showing various adjusted current values based on an initial target current value used during the example programming operation associated with FIG. 22. As shown in FIG. 23, the programming operation may initially use a signal with a pulse 2302 having an amplitude value corresponding to current value I_(RESET2) to reset the memory cell and successively adjust the amplitude of pulse 2302 to obtain adjusted pulses 2312, 2322, 2332, 2342, and 2352 of the signal with the adjusted amplitude values corresponding to adjusted current values I_(RESET2B), I_(RESET2C), I_(RESET2D), I_(RESET2E), and I_(RESET2F), respectively. Current values such as I_(RESET2), I_(RESET2B), I_(RESET2C), I_(RESET2D), I_(RESET2E), and I_(RESET2F) may be called programming parameter values. The programming operation may then use these programming parameter values and repeat the programming until the programmed resistance value (e.g., R_(PRG.F) in FIG. 22) is within the R_(TAR) range (FIG. 22).

Further, as shown in FIG. 23, the programming operation may use a signal 2300 having a pulse amplitude value corresponding to current value I_(SET) to program (e.g., set) the memory cell to an initial resistance value (e.g., R0) before each time the programming operation programs (e.g., resets) the memory cell that results in the memory cell having programmed resistance value R_(PRG.A), R_(PRG.B) R_(PRG.C), R_(PRG.D), R_(PRG.E), or R_(PRG.F). Setting the memory cell before each time it is reset may allow the memory cell to have a consistent initial resistance value (e.g., R0) to improve the programming operation.

FIG. 24 is a graph showing resistance versus current during an example programming operation using adjusted current values based on slope calculations according to an embodiment of the invention. R_(TAR) may represent a target resistance value to be programmed to a memory cell. R_(TAR) may correspond to one of R1, R2, and R3 of FIG. 20 and FIG. 21. For the purposes of describing FIG. 24, R_(TAR) is assumed to be resistance value R2. FIG. 24 also shows an R_(TAR) range, which may include resistance values within an upper resistance value equal to R_(TAR) plus an offset value OFFSETH, and a lower resistance value equal to R_(TAR) minus an offset value OFFSETL. R_(PRG.W), R_(PRG.X)/R_(PRG.Y), and R_(PRG.Z) may represent various programmed resistance values that the memory cell may have during various repeated programming to cause the programmed resistance (e.g., R_(PRG.Y)) value to be within the R_(TAR) range.

As described above with reference to FIG. 20, based on a relationship between resistance value and current value, a programming operation may use a signal with an initial target amplitude value corresponding to current value I_(RESET1)/I_(RESET2), or I_(RESET3) to reset the memory cell to resistance value R1, R2, or R3. Here, in the example of FIG. 24, since R_(TAR) is assumed to be resistance value R2, the programming operation may use a signal having a pulse with an initial target amplitude value corresponding to target current value I_(TAR)=I_(RESET2) to program the memory cell so that R_(TAR) (or R2 in this example) may be programmed into the memory cell. Unlike the programming operation associated with FIG. 22, the programming operation associated with FIG. 24 may adjust the current value based on slope calculations.

The programming operation associated with FIG. 24 may include the following activities. The programming operation may initially use current value (e.g., I_(RESET2)) to program the memory cell. If the programmed resistance value R_(PRG) is outside the R_(TAR) range, then the programming operation may successively adjust (e.g., by increasing or decreasing the current value by an adjusted current amount such as Δi1, Δi2, or Δi3) the current value based on slope calculations to obtain different adjusted current values. Adjusted current amounts such as Δi1, Δi2, and Δi3 may be called adjusted parameter amounts. After each adjustment, the programming operation may use the adjusted current values and repeat programming the memory cell until R_(PRG) (e.g., R_(PRG.Y) in FIG. 24) is within the R_(TAR) range.

For example, in FIG. 24, after being programmed using current value I_(RESET2), the memory cell may have R_(PRG.W), which is outside the R_(TAR) range and less than R_(TAR). The programming operation may increase I_(RESET2) to an adjusted current value I_(RESET2X)=I_(RESET2)Δi1 and repeat the programming.

After being programmed using I_(RESET2X), the memory cell may have R_(PRG.X), which is still outside the R_(TAR) range and less than R_(TAR). The programming operation may increase I_(RESET2X) to adjusted current value I_(RESET2Y)=I_(RESET2X)+Δi2 and repeat the programming.

After being programmed using I_(RESET2Y), the memory cell may have R_(PRG.Y), which is within the R_(TAR) range and less than R_(TAR). Since R_(PRG.Y) is within the R_(TAR) range, the programming operation may finish programming the memory cell.

In the above example, instead of the situation where the memory cell may have R_(PRG.W) after it is programmed using the initial I_(RESET2), the memory cell may have R_(PRG.X). As shown in FIG. 24, R_(PRG.Z) is outside the R_(TAR) range and greater than R_(TAR). In this situation, the programming operation may decrease I_(RESET2) to an adjusted current value I_(RESET2Y)=I_(RESET2)−Δi3 and repeat the programming. If the memory cell has R_(PRG.Y) after it is programmed using I_(RESET2Y), then the programming operation may finish programming the memory cell. Otherwise, the programming operation may repeat one or more times using other adjusted current values until R_(PRG) is within the R_(TAR) range.

As described above, the programming operation may increase the previous current value by an adjusted current amount (e.g., Δi1 or Δi2) if, after being programmed, the memory cell has a resistance value (e.g., R_(PRG.W) or R_(PRG.X)) outside the R_(TAR) range and less than R_(TAR). In the opposite case, the programming operation may decrease the previous current value by an adjusted current amount (e.g., Δi3) if after being programmed the memory cell has a resistance value (e.g., R_(PRG.Z)) outside the R_(TAR) range and greater than R_(TAR).

The programming operation may determine the value of adjusted current amounts Δi1, Δi2, and Δi3 based on slope calculations as follows.

FIG. 24 shows curve 2400 having a slope 2405 between two points R_(CUV1) and R_(CUV2), which may have values (resistance values) of two points that are relative closer to both sides of R_(TAR) on curve 2400. For example, if R_(TAR) is assumed to be R2, then points R_(CUV1) and R_(CUV2) may have values equal to resistance values R1 and R3, respectively. In another example, although FIG. 24 shows points R_(CUV1) and R_(CUV2) as being outside R_(TAR) range, points R_(CUV1) and R_(CUV2) however may have values equal to R_(TAR)+OFFSET_(H) and R_(TAR)−OFFSET_(L), respectively (two points where curve 2400 intersects lines represented by R_(TAR)+OFFSET_(H) and R_(TAR)−OFFSET_(L)). The programming operation may use the value of slope 2405 to determine the value of the adjusted current amount (e.g., Δi1, Δi2, or Δi3) that it may use during one or more repeated programming.

As shown in FIG. 24, after programming using I_(RESET2), the memory cell may have R_(PRG.W), which is still outside the R_(TAR) range and less than R_(TAR). Thus, as described above, the programming operation may increase I_(RESET2) by an adjusted current amount and repeat the programming. Since slope 2405 is calculated based on two points that are relatively closer to R_(TAR), using slope 2405 to calculate the adjusted current amount (e.g., Δi1) to be used in the next (repeated) programming may improve a chance of the next resistance value (e.g., R_(PRG.X) or R_(PRG.Y) resulted from the next programming) being within the R_(TAR) range. The programming operation may determine the value of Δi1 by equating the value of the slope of R_(PRG.W) and R_(TAR) (e.g., slope 2401 in FIG. 24) to the value of slope 2405 (slope₂₄₀₅). The value of slope 2405 and value of slope 2401 (slope₂₄₀₄) may follows expressions (1) and (2), respectively.

slope₂₄₀₅=(R _(CUV2) −R _(CUV1))/(I _(CUV2) −I _(CUV1))  (1)

slope₂₄₀₁ =ΔR _(i1) /Δi1=(R _(TAR) −R _(PRG.W))/Δi1  (2)

If slope₂₄₀₁=slope₂₄₀₅, then based on expressions (1) and (2),

(R _(TAR) −R _(PRG.W))/Δi1=(R _(CUV2) −R _(CUV1))/(I _(CUV2) −I _(CUV1))  (3)

Thus, based on expression (3), Δi1 may follow expression (4) or (5) below.

Δi1=(R _(TAR) −R _(PRG.W))*[(I _(CUV2) −I _(CUV1))/(R _(CUV2) −R _(CUV1))]  (4) or

Δi1=(R _(TAR) −R _(PRG.W))*(1/slope₂₄₀₅)  (5)

Since the values for each of the variables on the right side of expression (5) is known, the value of Δi1 may be determined.

In calculations similar to that of expression (5), the value of Δi2 and Δi3 may be determined by expressions (6) and (7) as follows.

Δi2=ΔR _(i2) /Δi2=(R _(TAR) −R _(PRG.X))*[(I _(CUV2) −I _(CUV1))/(R _(CUV2) −R _(CUV1))] or

Δi2=(R _(TAR) −R _(PRG.X))*(1/slope₂₄₀₅)  (6)

Δi3=ΔR _(i3) /Δi3=(R _(TAR) −R _(PRG.Z))*[(I _(CUV2) −I _(CUV1))/(R _(CUV2) −R _(CUV1))] or

Δi3=(R _(TAR) −R _(PRG.Z))*(1/slope₂₄₀₅)  (7)

Since the programmed resistance value (e.g., R_(PRG.W) or R_(PRG.X)) may have a value that is less than R_(TAR) or greater than R_(TAR) (e.g., R_(PRG.Z) is greater than R_(TAR)), and since a programming operation may increase or decrease the current value by a positive adjusted current value (e.g., Δi1, Δi2, or Δi3), expressions (5), (6), and (7) above may be generally expressed as Δix=|R_(TAR)−R_(PRG)|*(1/slope₂₄₀₅), which is absolute value of a difference between R_(TAR) and R_(PRG) times (1/slope₂₄₀₅), where Δix represents an adjusted current amount (e.g., Δi1, Δi2, or Δi3) to be used in a repeated programming, and R_(PRG) represents the programmed resistance value (e.g., R_(PRG.W), R_(PRG.X) or R_(PRG.Z)) that the memory cell may have after a previous programming. As described herein, since the adjusted current amount (Δi1, Δi2, or Δi3) may be different (e.g., varied) when the current value is adjusted (either increased or decreased), the adjusted current amount may include a variable current amount.

FIG. 25 is a graph showing various adjusted current values used during the example programming operation associated with FIG. 24. FIG. 25 shows two example situations that may occur during programming, one situation on the left side of line 2544 and another situation on the right side of line 2544. As shown on the left side of line 2544 in FIG. 25, the programming operation may initially use a signal with a pulse 2502 having an amplitude value corresponding to current value I_(RESET2). Then, the programming operation may successively adjust the amplitude of pulse 2502 to obtain adjusted pulses 2512 and 2522 of the signal with the adjusted amplitude values corresponding to adjusted current values I_(RESET2W) and I_(RESET2X), respectively. The programming operation may use these adjusted current values and repeat the programming until the programmed resistance value (e.g., R_(PRG.Y) in FIG. 24) is within the R_(TAR) range (FIG. 24). In the situation on the right side of line 2544 in FIG. 25, the programming operation may initially use a signal with a pulse 2502 having an amplitude value corresponding to current value I_(RESET2) and then successively adjust the amplitude of pulse 2502 to obtain at least one adjusted pulse, such as pulse 2532 of the signal, with the adjusted amplitude value corresponding to an adjusted current value, such as I_(RESET2Y). The programming operation may use the adjusted current value and repeat the programming until the programmed resistance value (e.g., R_(RPG.Y)) is within the R_(TAR) range (FIG. 24). Current values such as I_(RESET2), I_(RESET2X), and I_(RESET2Y) may be called programming parameter values.

Further, as shown in FIG. 25, the programming operation may use a signal 2500 having a pulse amplitude value corresponding to current value I_(SET) to program (e.g., set) the memory cell to an initial resistance value (e.g., R0) before each time the programming operation programs (e.g., resets) the memory cell, which results in the memory cell having a programmed resistance value R_(RPG.W), R_(RPG.X), or R_(RPG.Z).

The description above with reference to FIG. 20 through FIG. 25 describes examples in which a programming operation may adjust the amplitude of a pulse of a signal to program a memory cell until a programmed resistance of the memory cell is within a target resistance value range.

Alternatively, to program the memory cell, the programming operation may hold the amplitude value of the pulse at the same or substantially the same value and adjust a transition time value of the pulse (instead of adjusting the amplitude of the pulse or in addition to adjusting the amplitude of the pulse) until the programmed resistance of the memory cell is within the target resistance value range.

FIG. 26 is a graph of a curve 2610 showing a relationship between resistance values R0, R1, R2, and R3 and transition time values T₀, T₁, T₂, and T₃ for use as target values during a programming operation according to an embodiment of the invention. As shown in FIG. 26, each of transition time values T₀, T₁, T₂, and T₃ may represent a pulse fall time value of a pulse. The device associated with FIG. 26 may characterize (e.g., program and then read) selected memory cells to determine the relationship (e.g., resistance values R0, R1, R2, and R3 and pulse fall time values) represented by curve 2610 and then store (e.g., in the form of values in a table) the relationship in a storage area of the device. As shown in FIG. 26, the programming operation may use a signal with a pulse 2600 to program the memory cell to resistance value R0, and different signals with different pulses 2601, 2602, and 2603 with transition time values (in time unit such as nanosecond) T₁, T₂, and T₃ to program the memory cell to resistance value R1, R2, or R3, respectively. Transition time value T₀ of pulse 2600 may be greater than each of transition time values T₁, T₂, and T₃ to program the memory cell to a lower resistance value R0. As shown in FIG. 26, pulses 2600, 2601, 2602, and 2603 may have the same or substantially the same amplitude value (e.g., same current value). Each of transition time values T₀, T₁, T₂, and T₃ may be called a target transition time value (occasionally referred to as T_(TAR)). During a programming operation (e.g., after the initialization), the programming operation may select one of transition time values T₀, T₁, T₂, and T₃ to be T_(TAR), depending on which one the resistance values R0, R1, R2, and R3 is determined to be R_(TAR). During programming, the programming operation may successively adjust T_(TAR) to program the memory cell until the memory cell has a resistance value equal to R_(TAR) or to a value within an acceptable range of R_(TAR).

FIG. 27 is a graph showing different signals with pulses 2700, 2701, 2702, and 2703 having adjustable transition time values T₀, T₁, T₂, and T₃ used during a programming operation according to an embodiment of the invention. A programming operation may use pulses 2700, 2701, 2702, and 2703 to cause a memory cell to have corresponding resistance values R0, R1, R2, and R3, respectively. As shown in FIG. 27, pulses 2700, 2701, 2702, and 2703 may have the same or substantially the same amplitude value (e.g., same current value I_(RESETY)) Transition time values T₀, T₁, T₂, and T₃ of FIG. 27 may correspond to those of FIG. 26. Alternatively, pulse 2700 of FIG. 27 may have a square (or substantially square) wave shape and an amplitude value (e.g., current value I_(SET)) that is less than the amplitude of each of pulses 2701, 2702, and 2703 with sufficient pulse width to fully set the memory cell.

As shown in FIG. 27, the programming operation may adjust transition time values T₀, T₁, T₂, and T₃ to obtain adjusted transition time values T_(ADJ0), T_(ADJ1), T_(ADJ2), and T_(ADJ3), respectively. Each of adjusted transition time values T_(ADJ0), T_(ADJ1), T_(ADJ2), and T_(ADJ3) may include a value that is less than or greater than its corresponding transition time value by an adjusted time amount. For example, as shown in FIG. 27, adjusted transition time value T_(ADJ2) may be less than or greater than transition time value T₂. Depending on the value of R_(TAR), the programming operation may select one of transition time values T₁, T₂, and T₃ as an initial target transition time value and may adjust the selected initial target transition time value during programming to program the memory cell until its programmed resistance value R_(PRG) is equal to R_(TAR) (e.g., R1, R2, or R3) or is within an acceptable range of R_(TAR).

FIG. 28 is a graph showing resistance versus transition time during an example programming operation according to an embodiment of the invention. The example programming operation associated with FIG. 28 may include activities similar to the example programming operation associated with FIG. 22 with some exceptions. As describe above, the programming operation associated with FIG. 22 may successively increase the current value (e.g., I_(RESET2)) when R_(PRG) is outside the R_(TAR) range and less than R_(TAR) and decrease the current value when R_(PRG) is outside the R_(TAR) range and greater than R_(TAR). In FIG. 28, however, the programming operation may successively decrease the transition time value (e.g., T₂) when R_(PRG) is outside the R_(TAR) range and less than R_(TAR) and increase the transition time value when R_(PRG) is outside the R_(TAR) range and greater than R_(TAR).

In FIG. 28, R_(TAR), R_(TAR)+OFFSET_(H), R_(TAR)−OFFSET_(L), and R_(TAR) range may be similar to or identical to those described above with reference to FIG. 22. For the purposes of describing FIG. 28, R_(TAR) is assumed to be resistance value R2. R_(PRG.A), R_(PRG.B), R_(PRG.C), R_(PRG.D), R_(PRG.E), and R_(PRG.F) may represent various programmed resistance values that the memory cell may have during various repeated programming. The programming operation may repeat programming (e.g., resetting) the memory cell more than one times in an attempt to cause the programmed resistance value to be within the R_(TAR) range. In the example of FIG. 28, since R_(TAR) is assumed to be resistance value R2, the programming operation may use a signal having a pulse with an initial target transition time value T_(TAR)=T₂ to program the memory cell so that R_(TAR) (or R2 in this example) may be programmed into the memory cell.

The programming operation associated with FIG. 28 may include the following activities. The programming operation may initially use a transition time value (e.g., T₂) to program the memory cell. If the programming resistance value R_(PRG) is outside the R_(TAR) range, then the programming operation may successively adjust the transition time value (e.g., by increasing or decreasing the transition time value by an adjusted time amount such as Δt, ½ Δt, ¼ Δt, ⅛ Δt, or 1/16 Δt) to obtain different adjusted transition time values. Adjusted time amounts such as Δt, ½ Δt, ¼ Δt, ⅛ Δt, and 1/16 Δt may be called adjusted parameter amounts. After each adjustment, the programming operation may use the adjusted transition time values and repeat programming the memory cell until R_(PRG) is within the R_(TAR) range.

For example, in FIG. 28, after being programmed using a transition time value T₂, the memory cell may have R_(PRG.A), which is outside the R_(TAR) range and less than R_(TAR). Thus, the programming operation may adjust transition time value T₂ and repeat the programming until the memory cell has the programmed resistance value (e.g., R_(PRG.F)) that is within the R_(TAR) range.

As shown in FIG. 28, the memory cell may have R_(PRG.A), which is outside the R_(TAR) range, after being programmed using an initial transition time value T₂. To repeat programming, the programming operation may adjust transition time value T₂ by decreasing T₂ by Δt, and then repeat programming the memory cell using an adjusted transition time value T_(ADJ2B)=T₂−Δt.

After programming using T_(ADJ2B), the memory cell may have R_(PRG.B), which is still outside the R_(TAR) range and less than R_(TAR). Thus, the programming operation may adjust T_(ADJ2B) by decreasing T_(ADJ2B) by Δt, and then repeat programming the memory cell using an adjusted transition time value T_(ADJ2C)=T_(ADJ2B)−Δt. The programming operation may keep decreasing the current value by Δi until the programmed resistance value is either within the R_(TAR) range (then the programming operation may finish programming the memory cell) or outside the R_(TAR) range but greater than R_(TAR) (then the programming operation may continue but with an adjusted current amount less than Δi).

After programming using T_(ADJ2C), the memory cell may have R_(PRG.C), which is still outside the R_(TAR) range and less than R_(TAR). Thus, the programming operation may adjust T_(ADJ2C) by decreasing T_(ADJ2C) by Δt, and then repeat programming the memory cell using an adjusted transition time value T_(ADJ2D)=T_(ADJ2C)−Δt.

After programming using T_(ADJ2D), the memory cell may have R_(PRG.D), which is still outside the R_(TAR) range but greater than R_(TAR). Thus, the programming operation may adjust T_(ADJ2D) by increasing T_(ADJ2D) by ½ Δt, and then repeat programming the memory cell using an adjusted transition time value T_(ADJ2E)=T_(ADJ2D)+½ Δt.

After programming using T_(ADJ2E), the memory cell may have R_(PRG.E), which is still outside the R_(TAR) range and greater than R_(TAR). Thus, the programming operation may adjust T_(ADJ2E) by increasing T_(ADJ2E) by ¼ Δt, and then repeat program the memory cell using an adjusted transition time value T_(ADJ2F)=T_(ADJ2E)+¼ Δt. After programming using T_(ADJ2F), the memory cell may have R_(PRG.F), which is within the R_(TAR) range. Since R_(PRG.F) is within the R_(TAR) range, the programming operation may finish programming the memory cell.

As described above, the programming operation may decrease the previous transition time value by an adjusted transition time amount (e.g., Δt) if after being programmed, the memory cell has a programmed resistance value (e.g., R_(PRG.A), R_(PRG.B), or R_(PRG.C)) that is outside the R_(TAR) range and less than R_(TAR). In the opposite, the programming operation may increase the previous transition time value by an adjusted transition time amount (e.g., ½ Δt or ¼ Δt) if after being programmed, the memory cell has a resistance value (e.g., R_(PRG.D) or R_(PRG.E)) that is outside the R_(TAR) range and greater than R_(TAR). Since the adjusted transition time amount (e.g., Δt, ½ Δt, ¼ Δt, ⅛ Δt, or 1/16 Δt) may be different (e.g., varied) when the transition time is adjusted, the adjusted transition time amount may include a variable transition time amount.

In FIG. 28, Δt may be called a selected time amount. The device associated with FIG. 28 may select the value of Δt to be any appropriate value. However, to improve the chance of a programmed resistance to be within the R_(TAR) range or to reduce the number of repeated programming, for example, the device may select the value of Δt based on a difference in values between two adjacent transition time values (e.g., T₁, T₂, and T₃) from a resistance value versus transition time value curve, such as curve 2800 of FIG. 28. For example, the device may select the value of Δt to be equal to the difference between T_(TAR)+OFFSET″_(H) and T_(TAR)−OFFSET″_(L) or less than (e.g., one-half of) the difference between T_(TAR)+OFFSET″_(H) and T_(TAR)+OFFSET″_(L). T_(TAR)+OFFSET″_(H) and T_(TAR)−OFFSET″_(L) may include transition time values of the pulses that are used to obtain R_(TAR)+OFFSET_(H) and R_(TAR)−OFFSET_(L), respectively.

FIG. 29 is a graph showing various adjusted transition time values based on an initial target transition time value used during the example programming operation associated with FIG. 28. As shown in FIG. 29, the programming operation may initially use a signal with a pulse 2902 having an amplitude value corresponding to current value I_(RESETY), and a transition time value T₂ to program the memory cell and successively adjust the transition time T₂ of pulse 2902 to obtain adjusted pulses 2912, 2922, 2932, 2942, and 2952 of the signal with the adjusted transition time values T_(ADJ2B), T_(ADJ2C), T_(ADJ2D), T_(ADJ2E), and T_(ADJ2F), respectively. Transition time values such as T₂, T_(ADJ2B), T_(ADJ2C), T_(ADJ2D), T_(ADJ2E), and T_(ADJ2F) may be called programming parameter values. The programming operation may then use these programming parameter values and repeat the programming until the programmed resistance value (e.g., R_(PRG.Y) in FIG. 28) is within the R_(TAR) range (FIG. 28).

In the description herein, the features shown in drawings are not scaled. Some drawings may exaggerate or contrast the relationship among the features shown in the drawings for ease of illustrating the embodiments herein. For example, FIG. 22 shows the R_(TAR) range being greater than two immediate adjacent programmed resistance values (e.g., R_(PRG.B) and R_(PRG.C)). However, the R_(TAR) range may be less than the difference between two immediate adjacent programmed resistance values. Further, some examples herein may use some specific number of programmed resistance values (e.g., R_(PRG.A), R_(PRG.B), R_(PRG.C), R_(PRG.D), R_(PRG.E), and R_(PRG.F) in FIG. 22, FIG. 23, or FIG. 28, or R_(PRG.W), R_(PRG.X), R_(PRG.Y), and R_(PRG.Z) in FIG. 24) while programming a memory cell to a target resistance value. The number of programmed resistance values, however, may be less than or more than the number used in these examples. For example, in FIG. 22 and FIG. 28, the resistance value of the memory cell may jump from R_(PRG.A) to R_(PRG.D) after the programming operation adjusts the current amount by +Δi. In another example, R_(PRG.A) (FIG. 22 or FIG. 28), R_(PRG.W) (FIG. 24) may be within the R_(TAR) range after one programming such that no repeated programming and adjustment is performed. Thus, during a programming operation, the programmed resistance value of the memory cell may converge to the R_(TAR) range quicker than the examples described herein.

The above description with reference to FIG. 20 through FIG. 29 describes some example programming operations of a device including activities to adjust an amplitude value (e.g., current value) or a transition time value (e.g., pulse fall time value) of a pulse to program (e.g., reset) a memory cell. The description below with reference to FIG. 30 through FIG. 34 describes methods of operating a device that may include some or all activities of the above example programming operations.

FIG. 30 is a flow diagram showing methods 3000 of operating a device according to an embodiment of the invention. Methods 3000 may start at activity 3005, which may include turning on (e.g., powering up) or supplying power to the device.

Activity 3010 may include creating a table that a programming operation of the device may use during programming at least one memory cell of the device. Activity 3010 may create the table during an interval such as during device initialization without receiving a command (e.g., programming or read command) from an external device, such as a processor or a memory controller. The table may include programming parameter values and values representing characterization of the memory cells. For example, activity 3010 may include creating a table such as a table 3100 (FIG. 31). As shown in FIG. 31, table 3100 may include values associated with a relationship between target resistance values R0, Ra, R1, Rb, R2, Rc, R3, and Rd and corresponding current values I_(SET), Ia, I_(RESET1), Ib, I_(RESET2), Ic, I_(RESET3), and Id. Table 3100 may also include values associated with relationships between values of information (e.g., “00”, “01”, “10”, and “11”) and resistance values R0, R1, R2, and R3. The device, however, may exclude the relationship between resistance values R0, R1, R2, and R3 and the values of information (e.g., “00”, “01”, “10”, and “11”) from table 3100 and store the relationship in a location separate from table 3100. For simplicity, table 3100 includes an example of four resistance values, four current values, and four information values to indicate that each memory of the device may be configured to store four logic states (which may also be called levels), representing two bits of information. Table 3100, however, may include other numbers (e.g., eight, sixteen, or other) of resistance values, current values, and information values when the memory cell is configured to store other number of logic states representing other number of bits (e.g., three or four) of information.

To create the table 3100, activity 3010 may include programming (e.g., during device initialization) some selected memory cells of the memory device using signals with different amplitude values corresponding to different current values (e.g., I_(SET), Ia, I_(RESET1), Ib, I_(RESET2), Ic, I_(RESET3), and Id). A different amplitude value used to program a selected memory cell may result in the selected memory cell having a different resistance value. After the selected memory cells are programmed, activity 3010 may read the selected memory cells and measure their resistance values. Then, activity 3010 may store in table 3100 the measured resistance values (e.g., R0, Ra, R1, Rb, R2, Rc, R3, and Rd) and the corresponding current values (e.g., I_(SET), Ia, I_(RESET1), Ib, I_(RESET2), Ic, I_(RESET3), and Id) that have been used to program the selected memory cells to the measured resistance values. Methods 3000 may use one of the current values in table 3100 or extrapolated values based on table 3100 as an initial (e.g., target) current value (e.g., I_(SET)/I_(RESET1), I_(RESET2), or I_(RESET3)) to program a memory cell during a programming operation, e.g., a programming operation in response to a programming command provided from a device such as a processor or a memory controller.

Alternatively or additionally, activity 3010 of FIG. 30 may include creating a table such as a table 3200 of FIG. 32. As shown in FIG. 32, table 3200 may include relationships between target resistance values R0, Ra, R1, Rb, R2, Rc, R3, and Rd and corresponding transition time values T₀, Ta, T₁, Tb, T₂, Tc, T₃, and Td of a pulse of signals that the programming operation used to program the memory cells to resistance values such as R0, Ra, R1, Rb, R2, Rc, R3, and Rd. Table 3200 may also include a relationship between values of information (e.g., “00”, “01”, “10”, and “11”) and resistance values R0, R1, R2, and R3. The device, however, may exclude the relationship between resistance values R0, R1, R2, and R3 and the values of information (e.g., “00”, “01”, “10”, and “11”) from table 3200 and store the relationship in a location separate from table 3200.

To create the table 3200, activity 3010 may include programming (e.g., during device initialization) some selected memory cells of the memory device using signals with different transition time values (e.g., T₀, Ta, T₁, Tb, T₂, Tc, T₃, and Td). After the selected memory cells are programmed, activity 3010 may read the selected memory cells and measure the resistance values of the memory cells. Then, activity 3010 may store in table 3200 the measured resistance values (e.g., R0, Ra, R1, Rb, R2, Rc, R3, and Rd) and the corresponding transition time values (e.g., T₀, Ta, T₁, Tb, T₂, Tc, T₃, and Td) that have been used to program the selected memory cells. Methods 3000 may use one of the transition time values in table 3200 or extrapolated values based on table 3200 as an initial (e.g., target) transition time value (e.g., T₀, T₁, T₂, or T₃) to program a memory cell during a programming operation, e.g., a programming operation in response to a programming command provided from a device such as a processor or a memory controller.

Each of table 3100 and 3200 may include hardware, firmware, software, or a combination thereof. The device used by methods 3000 may store the table created by activity 3010 in a storage area similar to or identical to storage area 144 of FIG. 1. Methods 3000 of FIG. 30 may use either table 3100 or 3200 to program memory cells of the device. After activity 3010 creates the table (e.g. table 3100 or 3200), method 3000 may continue with activity 3015.

Activity 3015 may include acquiring programming parameter values used to program the memory cell. For example, based on a value of information to be programmed to the memory cell, acquiring programming parameters in activity 3015 may include accessing some locations of the device including a table, such as table 3100 or 3200, to determine a target resistance value R_(TAR), and either a target amplitude value (e.g., I_(TAR)) or a target transition time value (e.g., T_(TAR)) based on the target resistance value. Activity 3015 may also include determining a target resistance value range (R_(TAR) range) based on R_(TAR). For example, activity 3015 may include determining upper and lower resistance values of the R_(TAR) range based on R_(TAR) and offset values such as OFFSET_(H) and OFFSET_(L) similar or identical to those described above with reference to FIG. 20 through FIG. 29. A storage area of the device may store the offset values.

Activity 3025 in FIG. 30 may program the memory cell to cause it to have an initial resistance value such as a “set” resistance value represented by R0 of FIG. 23, FIG. 25, or FIG. 29. Activity 3025 may include the set activities as described above with reference to FIG. 18 and FIG. 19 to change the material of a programmable portion of the memory cell to a crystalline phase (e.g., crystalline phase 613 of FIG. 6).

Activity 3030 may include programming the memory cell to cause it to have a programmed resistance value R_(PRG) equal to R_(TAR) or within the R_(TAR) range. Activity 3030 may use a signal with at least one pulse, the pulse may have an amplitude value, or transition time value, or both, similar to or identical to those described above with reference to FIG. 20 through FIG. 29. In activity 3030, programmed resistance value R_(PRG) may have a value similar to or identical to one or more of R_(PRG.A), R_(PRG.B), R_(PRG.C), R_(PRG.D), R_(PRG.E), and R_(PRG.F) of FIG. 22 and FIG. 28 or one or more of R_(PRG.W), R_(PRG.X), R_(PRG.Y), and R_(PRG.Z) of FIG. 24.

Activity 3035 of FIG. 30 may include sensing the memory cell to obtain a resistance value (e.g., the resistance value programmed in activity 3025 or 3030, or both). Methods 3000 may, however, include skipping activities 3025 and 3030 (indicated by activity 3034) and may sense the memory cell to obtain a resistance value without programming the memory cell in activities 3025 and 3030. The memory cell may already have an existing resistance value (before activities 3025 or 3030 are performed), which may be within the R_(TAR) range. Thus, skipping programming activities 3025 and 3030 may avoid an additional programming of the memory cell, thereby improving the life of the device. In this description, reading a memory cell is one form of sensing the memory cell.

Activity 3040 may include determining whether the programmed resistance value of the memory cell is outside the R_(TAR) range. If the programmed resistance value is not outside the R_(TAR) range (i.e., within the R_(TAR) range), then methods 3000 may continue with activity 3045 to finish the programming. If the programmed resistance value is outside the R_(TAR) range, then methods 3000 may continue to activity 3099. If methods 3000 skip activities 3025 and 3030, activity 3040 may include determining whether the existing resistance value of the memory cell is outside the R_(TAR) range. If the existing resistance value is not outside the R_(TAR) range (i.e., within the R_(TAR) range), then methods 3000 may continue with activity 3045 to finish the programming. If the existing resistance value is outside the R_(TAR) range, then methods 3000 may continue to activity 3099.

Activity 3099 may include adjusting one or both of the amplitude value of the pulse and the transition time value of the pulse used in activity 3099. For example, activity 3099 may include adjusting a current value by an adjusted current amount such as Δi (described with reference to FIG. 20 through FIG. 25). Alternatively or additionally, activity 3099 may include adjusting a transition time value by an adjusted time amount such as Δt (described with reference to FIG. 26 through FIG. 29). Methods 3000 may repeat some or all of activities 3088 and activity 3099 until programmed resistance value R_(PRG) of the memory cell is within the R_(TAR) range. For example, if methods 3000 uses table 3100 (resistance values and amplitude current values) to program the memory cell in activity 3088, then methods 3000 may adjust the current value in activity 3099 and repeat activities 3025, 3030, 3035, and 3040 (indicated by repeated activity 3055). In another example, if methods 3000 uses table 3200 (resistance values and transition time values) to program the memory cell in activity 3088, then methods 3000 may adjust the transition time value in activity 3099 and repeat activities 3030, 3035, and 3040 (indicated by activity 3033).

If activities 3025 and 3030 are skipped, and if the resistance value in activity 3040 is outside the R_(TAR) range, methods 3000 may jump from activity 3040 to activity 3025, omitting activity 3099 for one time, because an amplitude value may have not yet been used to program the memory.

The adjustments performed by activity 3099 may include activities of methods 3300 of FIG. 33 or activities of methods 3400 of FIG. 34.

FIG. 33 is a flow diagram showing methods 3300 of operating a device including amplitude value adjustment according to an embodiment of the invention. Methods 3300 may include activities 3388, which may be similar to identical to activities 3088 of FIG. 30. For example, activities 3388 may include activity 3302 to program a memory cell and activity 3304 to determine whether a programmed resistance value R_(PRG) of the memory cell is outside a resistance target value range (R_(TAR) range). FIG. 33 shows an example where activity 3304 may determine that R_(PRG) is outside the R_(TAR) range. Methods 3300 may also include activities 3399 to adjust the current value used to program the memory cell, and repeat the programming at activity 3302 until R_(PRG) is within the R_(TAR) range.

Methods 3300 may include activity 3305 to compare R_(PRG) with a target resistance value R_(TAR) if R_(PRG) is outside the R_(TAR) range. Based on the comparison, methods 3300 may continue with activity 3306 and either activity 3310 or 3320. Activity 3306 may conditionally modify Δi so that appropriate value of Δi may be used for activity 3310 or 3320. Activity 3306 is described in more details below. Activity 3310 may include increasing the current value by Δi if R_(PRG) is less than R_(TAR). Activity 3320 may include decreasing the current value by Δi if R_(PRG) is greater than R_(TAR). The current value may have an initial value corresponding to current value I_(RESET1), I_(RESET), or I_(RESET3) based on a table such as table 3100 of FIG. 31. The Δi may have an initial value selected in ways similar to or identical to those described above with reference to FIG. 22.

Activity 3306 may conditionally modify Δi such that Δi used in activity 3310 or 3320 may remain at the same value (e.g., a value equal to a previous value of Δi such as the initial value of Δi) or may vary, depending on the various values of R_(PRG). During the programming operation, R_(PRG) may have one or more values that are outside the R_(TAR) range and less than R_(TAR) (e.g., value similar to R_(PRG.A) or R_(PRG.B) of FIG. 22) and one or more other values that are outside the R_(TAR) range and greater than R_(TAR) (e.g., value similar to R_(PRG.C), R_(PRG.D), or R_(PRG.E) of FIG. 22). Methods 3300 may store the values of R_(PRG) during the programming operation to allow activity 3306 to conditionally modify Δi based on the stored values of R_(PRG), as described below.

Activity 3306 may keep Δi at the same value (e.g., a value equal to the initial value of Δi) if at any time during the programming operation all of the values of R_(PRG) are outside the R_(TAR) range and less than R_(TAR).

Activity 3306 may also keep Δi at the same value (e.g., a value equal to the initial value of Δi) if at any time during the programming operation all of the values of R_(PRG) are outside the R_(TAR) range and greater than R_(TAR).

Activity 3306 may modify Δi (e.g., decrease Δi) if at any time during the programming operation at least one value of R_(PRG) (e.g., value similar to R_(PRG.A) or R_(PRG.B) of FIG. 22) is outside the R_(TAR) range and less than R_(TAR), and at least one value of R_(PRG) (e.g., value similar to R_(PRG.C), R_(PRG.D), or R_(PRG.E) of FIG. 22) is outside the R_(TAR) range and greater than R_(TAR).

Based on the comparison in activity 3305 and the conditional modification of Δi in activity 3306, activity 3310 may increase the current value (used for the next programming sequence) by the same initial value of Δi if at any time during the programming operation all of the values of R_(PRG) are less than R_(TAR). Activity 3310 may increase the current value by a modified value of Δi if at any time during the programming operation at least one value of R_(PRG) is less than R_(TAR), and at least one value of R_(PRG) is greater than R_(TAR).

Based on the comparison in activity 3305 and the conditional modification of Δi in activity 3306, activity 3320 may decrease the current value (used for the next programming sequence) by the same initial value of Δi if at any time during the programming operation all of the values of R_(PRG) are greater than R_(TAR). Activity 3320 may decrease the current value by a modified value of Δi if at any time during the programming operation at least one value of R_(PRG) is less than R_(TAR), and at least one value of R_(PRG) is greater than R_(TAR).

Methods 3300 may modify Δi by reducing it by a factor of (½^(n)) where “n” is a real number. Methods 3300 may increase “n” by a non-integer value after each time activity 3306 modifies Δi so that the value of Δi may be different (e.g., reduced) when methods 3300 increases or decreases the current value. Methods 3300 may also increase “n” by an integer value (1, 2, or 3, or other value) after each time activity 3306 modifies Δi. For example, “n” may initially be equal to one before activity 3306 modifies Δi. Methods 3300 may increase “n” by one after each time activity 3306 modifies Δi. Thus, methods 3300 may increase or decrease an adjusted current value by an adjusted current amount of ½ Δi (i.e., (½¹) Δi=½ Δi) when activity 3306 modifies Δi for the first time. Then, method 3300 may increase or decrease other adjusted current values by (½^(n)) Δi, where “n” is an integer value corresponding to the total number of times that activity 3306 modifies Δi. Activity 3306 may modify Δi in ways similar to or identical to those described above with reference to FIG. 22 and FIG. 23 to get different adjusted current amount such as ½ Δi, ¼ Δi, ⅛ Δi, and 1/16 Δi.

Activity 3306 may modify Δi using slope calculations if methods 3300 use Δi having values similar to or identical to those of Δi1, Δi2, and Δi3 described above with reference to FIG. 24 and FIG. 25.

Methods 3300 may repeat activities 3388 and 3399 until R_(PRG) is within the R_(TAR) range.

FIG. 34 is a flow diagram showing methods 3400 of operating a device including transition time value adjustment according to an embodiment of the invention. Methods 3400 are similar to methods 3300 of FIG. 33 except that methods 3400 may increase or decrease a transition time value (instead of the current value as in FIG. 33) of a pulse. In FIG. 34, methods 3400 may include activities 3488, which may be similar to identical to activities 3088 of FIG. 30. For example, activities 3488 may include activity 3402 to program a memory cell, and activity 3404 to determine whether a programmed resistance value R_(PRG) is outside a resistance target value range (R_(TAR) range). FIG. 34 shows an example where activity 3404 may determine that R_(PRG) is outside the R_(TAR) range. Methods 3400 may also include activities 3499 to adjust the transition time value used to program the memory cell, and repeat the programming at activity 3402 until R_(PRG) is within the R_(TAR) range.

Methods 3400 may include activity 3405 to compare R_(PRG) with a target resistance value R_(TAR) if R_(PRG) is outside the R_(TAR) range. Based on the comparison, methods 3400 may continue with activity 3406 and either activity 3410 or 3420. Activity 3406 may conditionally modify Δt so that appropriate value of Δt may be used for activity 3410 or 3420. Activity 3406 is described in more details below. Activity 3410 may include decreasing the transition time value by Δt if R_(PRG) is less than R_(TAR). Activity 3420 may include increasing the transition time value by Δt if R_(PRG) is greater than R_(TAR). The transition time value may have an initial value corresponding to transition time value T₁, T₂, and T₃ based on a table such as table 320 of FIG. 32 The Δt may have an initial value selected in ways similar to or identical to those described above with reference to FIG. 28

Activity 3406 may conditionally modify Δt such that Δt used in activity 3410 or 3420 may remain at the same value (e.g., a value equal to a previous value of Δt such as the initial value of Δi) or may vary, depending on the various values of R_(PRG). During the programming operation, R_(PRG) may have one or more values that are outside the R_(TAR) range and less than R_(TAR) (e.g., value similar to R_(PRG.A), R_(PRG.B), or R_(PRG.C) of FIG. 28) and one or more other values that are outside the R_(TAR) range and greater than R_(TAR) (e.g., value similar to R_(PRG.D) or R_(PRG.E) of FIG. 28). Methods 3400 may store the values of R_(PRG) during the programming operation to allow activity 3406 to conditionally modify Δt based on the stored values of R_(PRG), as described below.

Activity 3406 may keep Δt at the same value (e.g., a value equal to the initial value of Δi) if at any time during the programming operation all of the values of R_(PRG) are outside the R_(TAR) range and less than R_(TAR).

Activity 3406 may also keep Δt at the same value (e.g., a value equal to the initial value of Δi) if at any time during the programming operation all of the values of R_(PRG) are outside the R_(TAR) range and greater than R_(TAR).

Activity 3406 may modify Δt (e.g., increase Δt) if at any time during the programming operation at least one value of R_(PRG) (e.g., value similar to R_(PRG.A), R_(PRG.B), or R_(PRG.C) of FIG. 28) is outside the R_(TAR) range and less than R_(TAR), and at least one value of R_(PRG) (e.g., value similar to R_(PRG.D) or R_(PRG.E) of FIG. 28) is outside the R_(TAR) range and greater than R_(TAR).

Based on the comparison in activity 3405 and the conditional modification of Δt in activity 3406, activity 3410 may decrease the transition time value (used for the next programming sequence) by the same initial value of Δt if at any time during the programming operation all of the values of R_(PRG) are less than R_(TAR). Activity 3410 may decrease the transition time value by a modified value of Δt if at any time during the programming operation at least one value of R_(PRG) is less than R_(TAR), and at least one value of R_(PRG) is greater than R_(TAR).

Based on the comparison in activity 3405 and the conditional modification of Δt in activity 3406, activity 3420 may increase the transition time value (used for the next programming sequence) by the same initial value of Δt if at any time during the programming operation all of the values of R_(PRG) are greater than R_(TAR). Activity 3420 may increase the transition time value by a modified value of Δt if at any time during the programming operation at least one value of R_(PRG) is less than R_(TAR), and at least one value of R_(PRG) is greater than R_(TAR).

Methods 3400 may modify Δt in ways similar to or identical to those by which methods 3300 of FIG. 33 modifies Δi. For example, methods 3400 may modify Δt by reducing it by a factor of (½^(n)), where “n” is a real number, in which methods 3400 may increase “n” by a non-integer value or by an integer value each time activity 3406 modifies Δt.

Activity 3406 may modify Δt using slope calculations described above with reference to FIG. 24 and FIG. 25 but with Δt replacing Δi1, Δi2, or Δi3 in the slope calculations. When modifying Δt using slope calculations, activity 3406 may use a curve similar to curve 2400 of FIG. 24 to calculate the slope but with a time axis replacing the current axis in FIG. 24 so that the curve used in the slope calculation may represent the relationship between resistance and transition time (similar to the relationship between resistance and transition time of curve 2800 of FIG. 28).

Methods 3400 may repeat activities 3488 and 3499 until R_(PRG) is within the R_(TAR) range.

The memory cell used in the programming operations and methods 3000, 3300, and 3400 described herein may include memory cell 600 or 1100 (FIG. 6 through FIG. 9 and FIG. 11 through FIG. 14). The programming operations described herein may allow a memory cell (e.g., memory cell 600 or 1100) to be configured with a relatively greater margin for current values along the current axis, such as the current axis of FIG. 20, or allow a relatively greater margin for transition time values along the time axis, such as the time axis of FIG. 26. A greater margin of current or transition time value may allow improved programming, more logic states to be stored in a memory cell, and/or tighter resistance distribution among the memory cells. Thus, higher bit density per memory cell may be achieved. The programming operations described herein may also improve data retention and device reliability. Further, the programming operations described herein may allow a relatively faster convergence to a target resistance value and/or relatively more consistent programmed resistance values, thereby device performance (e.g., programming speed) may also be improved.

The illustrations of the apparatus (e.g., memory devices 101 and 201, and memory cells 100, 200, 300, 400, 500, 600, and 1100) are intended to provide a general understanding of the structure of various embodiments and are not intended to provide a complete description of all the elements and features of the apparatus and systems that might make use of the operations and structures described herein.

Any of the features described above can be implemented in a number of ways, including simulation via software. Thus, the apparatus (e.g., a portion of memory device 101 or the entire memory device 101, and portion of memory device 201 or the entire memory device 201) described above may be characterized as “modules” (or “module”) herein. Such modules may include hardware circuitry, single and/or multi-processor circuits, memory circuits, software program modules and objects and/or firmware, and combinations thereof, as desired by the architect of the apparatus (e.g., memory devices 101 and 201), and as appropriate for particular implementations of various embodiments. For example, such modules may be included in a system operation simulation package, such as a software electrical signal simulation package, a power usage and distribution simulation package, a capacitance-inductance simulation package, a power/heat dissipation simulation package, a signal transmission-reception simulation package, and/or a combination of software and hardware used to operate or simulate the operation of various potential embodiments.

The methods and apparatus of various embodiments may include or be included in electronic circuitry used in high-speed computers, communication and signal processing circuitry, single or multi-processor modules, single or multiple embedded processors, multi-core processors, data switches, and application-specific modules including multilayer, multi-chip modules. Such methods and apparatus may further be included as sub-components within a variety of electronic systems, such as televisions, cellular telephones, personal computers (e.g., laptop computers, desktop computers, handheld computers, tablet computers, etc.), workstations, radios, video players, audio players (e.g., MP3 (Motion Picture Experts Group, Audio Layer 3) players), vehicles, medical devices (e.g., heart monitor, blood pressure monitor, etc.), set top boxes, and others.

One or more embodiments described herein include methods and apparatus having a module configured to program the memory cell using a signal to cause the memory cell to have a programmed resistance value, to adjust a programming parameter value of the signal if the programmed resistance value is outside a target resistance value range, and to repeat at least one of the programming and the adjusting if the programmed resistance value is outside the target resistance value range, the signal including a different programming parameter value each time the programming is repeated. Other embodiments, including additional apparatus and methods, are described above with reference to FIG. 1 through FIG. 34.

The above description and the drawings illustrate some embodiments of the invention to enable those skilled in the art to practice the embodiments of the invention. Other embodiments may incorporate structural, logical, electrical, process, and other changes. In the drawings, like features or like numerals describe substantially similar features throughout the several views. Examples merely typify possible variations. Portions and features of some embodiments may be included in, or substituted for, those of others. Many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. Therefore, the scope of various embodiments of the invention is checked by the appended claims, along with the full range of equivalents to which such claims are entitled.

The Abstract is provided to comply with 37 C.F.R. §1.72(b) requiring an abstract that will allow the reader to quickly ascertain the nature and gist of the technical disclosure. The Abstract is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 

What is claimed is:
 1. A method comprising: programming a memory cell using a first signal to cause the memory cell to have an initial resistance value; programming the memory cell using a second signal to cause the memory cell to have a programmed resistance value; adjusting a programming parameter value of the second signal if the programmed resistance value is outside a target resistance value range; and if the programmed resistance value is outside the target resistance value range, repeating at least one of programming the memory cell using the first signal, programming the memory cell using the second signal, and adjusting the programming parameter value of the second signal, wherein the second signal includes a different programming parameter value each time the programming is repeated.
 2. The method of claim 1, wherein adjusting the programming parameter value includes adjusting a pulse fall time value of the second signal.
 3. The method of claim 2, wherein adjusting includes increasing the pulse fall time value by a first adjusted time amount if the programmed resistance value is greater than a target resistance value, the target resistance value being a value within the target resistance value range, and wherein adjusting includes decreasing the pulse fall time value by the first adjusted time amount if the programmed resistance value is less than a target resistance value.
 4. The method of claim 1, wherein adjusting includes increasing the pulse fall time value by a second adjusted time amount less than the first time amount if the programmed resistance value before the repeating is less than the target resistance value and at least one of the programmed resistance value after the repeating is greater than the target resistance.
 5. The method of claim 1, wherein adjusting includes decreasing the fall time value by a second adjusted time amount less than the first time amount if the programmed resistance value before the repeating is greater than the target resistance value and at least one of the programmed resistance value after the repeating is less than the target resistance.
 6. The method of claim 2, wherein adjusting includes: increasing the pulse fall time value by a first adjusted time amount to obtain a first adjusted pulse fall time value for use in a first repeated programming if a first programmed resistance value is greater than a target resistance value, the target resistance value being a value within the target resistance value range, wherein the first programmed resistance value corresponds to the programmed resistance value of the memory cell before the first repeated programming is performed; and decreasing the first adjusted pulse fall time value by a second adjusted time amount to obtain a second adjusted pulse fall time value for use in a second repeated programming if a second programmed resistance value is less than the target resistance value, wherein the second programmed resistance value corresponds to the programmed resistance value of the memory cell after the first repeated programming is performed, and wherein the first adjusted time amount is greater than the second adjusted time amount.
 7. The method of claim 2, wherein adjusting includes: decreasing the pulse fall time value by a first adjusted time amount to obtain a first adjusted pulse fall time value for use in a first repeated programming if a first programmed resistance value is less than a target resistance value, the target resistance value being a value within the target resistance value range, wherein the first programmed resistance value corresponds to the programmed resistance value of the memory cell before the first repeated programming is performed; and increasing the first adjusted pulse fall time value by a second adjusted time amount to obtain a second adjusted pulse fall time value for use in a second repeated programming if a second programmed resistance value is greater than the target resistance value, wherein the second programmed resistance value corresponds to the programmed resistance value of the memory cell after the first repeated programming is performed, and wherein the first adjusted time amount is greater than the second adjusted time amount.
 8. The method of claim 2, comprising: selecting the pulse fall time value from among a first pulse fall time value associated with a first target resistance value, a second pulse fall time value associated with a second target resistance value, and a third pulse fall time value associated with a third target resistance value, wherein the target resistance value range includes one of the first, second, and third target resistance values.
 9. The method of claim 2, wherein adjusting the pulse fall time value includes one of increasing and decreasing the pulse fall time value by an adjusted time amount, and wherein the adjusted time amount is a function of a slope of a resistance versus transition time curve of the memory cell and a difference between a target resistance value and the programmed resistance value, the target resistance value being a value within the target resistance value range.
 10. The method of claim 9, wherein the slope is calculated based on a first point and a second point of the curve, wherein the first point corresponds to a resistance value greater than the target resistance value, and wherein the second point corresponds to a resistance value less than the target resistance value.
 11. The method of claim 9, wherein increasing the pulse fall time value by the adjusted time amount is performed if the programmed resistance value is greater than the target resistance value, and wherein decreasing the pulse fall time value by the adjusted time amount is performed if the programmed resistance value is less than the target resistance value. 